1. Field of the Invention
The present invention relates to an image forming method and apparatus, such as a copier, a facsimile machine, and a printer, and more particularly to a color image forming method and apparatus according to a dry two-component development system capable of collecting toners from image carrying members or an intermediate transfer member by using cleaning devices and of recycling the collected toners.
2. Discussion of the Background Arts
In a background image forming apparatus according to an electrographic system, optical image data is formed on an image carrying member uniformly charged, and an electrostatic latent image is formed thereon. The electrostatic latent image is developed into a visible image by toner supplied from a development device. The visible image is transferred to an intermediate transfer member and then to a recording medium (e.g., a transfer sheet). Alternatively, the visible image may be directly transferred to the recording medium. The visible image thus transferred is fixed on the recording medium. Accordingly, a toner image is formed on the recording medium.
In a background color image forming apparatus, toner images of four colors are formed on image carrying members and superimposed on an intermediate transfer member or on a recording medium conveyed by a recording medium conveying member such as a transfer belt. Accordingly, a full-color image is formed on the recording medium.
The background color image forming apparatus is either one of following two types. In a first type of color image forming apparatus, the toner images of four colors are sequentially formed on a single image carrying member or two image carrying members. In a second type of color image forming apparatus, an image carrying member, a development device which develops a toner image on the image carrying member, and a cleaning device which removes toner remaining on the image carrying member are provided for each of the four toner colors. The second type of color image forming apparatus usually uses a tandem system, according to which the image carrying members for the respective toner colors are arranged in a line along a moving direction of the intermediate transfer member or the transfer belt. Since the tandem-system color image forming apparatus is capable of forming the toner images of four colors at one time, the color image forming apparatus is operated at a relatively high speed.
Unlike a monochrome image forming apparatus, the color image forming apparatus uses a plurality of toner colors and frequently outputs a document, such as a photo image document, a relatively large space of which is occupied by an image. To reduce resources used by the color image forming apparatus as well as space and running cost of the apparatus, recycling of used toner has been an important issue to be addressed.
In the above-described background color image forming apparatus according to the electrographic system, the toner images of four colors formed on the respective four image carrying members are directly or indirectly superimposed on and transferred to the recording medium, so that a color image is formed thereon. After a transfer operation, a part of toner in the toner image carried by each of the image carrying members remains on the image carrying member without having been transferred to the recording medium. This toner remaining on the image carrying member is removed and collected by the cleaning device provided for each of the image carrying members.
Recycling of the collected toner, however, has a few problems. One of the problems is mixture of toners of different colors caused when a toner of one color stored at an upstream position is mixed into a toner of another color stored at a downstream position in a moving direction of the intermediate transfer member or the transfer belt.
In a tandem-system color image forming apparatus in which an image carrying member is provided for each of different toner colors, a cleaning device collects toner remaining on its corresponding image carrying member. It is therefore relatively easy to collect and recycle the remaining toner for future image forming operations. Further, since image forming systems for the respective toner colors are independent from one another, the mixture of toners of different colors may be considered preventable in the tandem-system color image forming apparatus. In reality, however, the mixture of toners of different colors occurs in a process of transferring toner images of the respective colors from the image carrying members to the recording medium.
Mechanism of mixing of different color toners is described below.
In the tandem-system color image forming apparatus, a color image is formed on the recording medium by, for example, transferring the toner images of the respective colors from the image carrying members to the recording medium conveyed by the transfer belt. In this case, by the time when a second toner in a second toner image carried by a second image carrying member is transferred to the recording medium, for example, the recording medium has already been covered with a first toner in a first toner image carried by a first image carrying member, which is located at an upstream position of the second image carrying member in a moving direction of the recording medium. The first toner transferred to the recording medium is transferred back to the second image carrying member in a transfer process of the second toner, and collected by a second cleaning device provided for the second image carrying member. In this way, the collected first toner mixes into the collected second toner.
In the background color image forming apparatus, therefore, if toner is collected from an image carrying members and directly sent back to a development device for the image carrying member, hue of toner contained in the development device gradually changes over time.
To solve this problem, toner of one color collected and mixed with toner of another color is usually mixed with a black toner for recycling or disposed as waste toner. If all of such toner is mixed into the black toner, however, it becomes difficult to maintain a balance between an amount of toner consumed and an amount of toner supplied to the development device. In an image forming operation of a photo image, for example, a relatively small amount of the black toner may be used. As a result, an amount of toner collected for recycling (i.e., recycled toner) in a cleaning process and supplied to the development device may exceed the amount of toner actually consumed.
To prevent the mixture of toners of different colors, there is an attempt to detect a mixing ratio of different color toners in developer contained in a development device by using a reflectance ratio detector, when recycled toner collected from an image carrying member in the cleaning process is supplied again to the development device for the image carrying member. Accordingly, a ratio between a new toner and the recycled toner to be supplied to the development device is set based on the detected mixing ratio of the different color toners.
There is another attempt to collect toners of the respective colors in one place, and mix and supply the collected toners to a black toner development device.
To prevent toner from being transferred from the recording medium back to the image carrying member, which is caused by inversely charged toner in the toner image formed on the recording medium, there is another attempt to remove electricity from a non-image area. In this attempt, electricity is removed to such a level that toner in a toner image does not scatter before the toner image is transferred.
To address toner aggregation and reduced chargeability of the toners collected for recycling in the cleaning process, another attempt has been made to adjust image forming process conditions by determining a ratio of the recycled toner to be supplied to the development device.